1. Field of the Invention
The present invention relates to a DC power source unit that can selectively charge a battery pack, which is used as a power source for a cordless power tool, and supply DC voltage to the cordless power tool through an adaptor shaped like the battery pack. More particularly, the invention relates to a DC power source unit that can accurately detect the fully charged condition of the battery pack.
2. Description of the Related Art
Cordless power tools are convenient because they can be used anywhere without being restricted by the length of a power cable. However, whenever the battery runs low, the battery pack needs to be removed and charged or else replaced with another battery pack that has already been charged.
A DC power source unit that converts alternating current to direct current can be used along with the battery pack. That is, the DC power source unit can be used when the cordless power tool is to be operated in a single place that has a source of alternating current, and the battery pack can be used when the cordless power tool is to be operated at several different places that have no source of alternating current.
However, this situation has a problem in that the operator of the cordless power tool must carry both the DC power source and the battery pack""s charging unit to the work area. Japanese Patent Publication No. 2000-184614(A) discloses a DC power source unit with a charging function. When the DC power source unit detects that the attached cordless power tool is being operated, it supplies power only to the cordless power tool and does not supply power to the battery pack for charging purposes. On the other hand, when DC power source unit detects that the cordless power tool is not being operated, then it charges a battery pack. That is, priority is given to supplying power to the cordless power tool and charging the battery pack is performed only when the cordless power tool is not being operated.
According to the DC power source unit of Japanese Patent Publication No. 2000-184614(A), detection of the battery pack""s fully charged condition is performed only when the battery pack is being charged. When the cordless power tool is turned ON immediately before the battery pack is fully charged, the DC power source unit is switched to a tool driving mode and charging the battery pack is interrupted so that detection of the battery pack""s fully charged condition is also interrupted. The detection of the fully charged condition will not be performed until the operations of the cordless power tool are stopped and charging the battery pack is resumed. Even if charging the battery pack is resumed, the fully charged condition cannot be detected immediately but detection of the fully charged condition will be made after the battery pack has actually reached the fully charged condition. This inaccurate detection of the fully charged condition is a problem with the conventional device.
More specifically, detection of the battery pack""s fully charged condition is performed based on change of the battery voltage and/or change of the battery temperature. To obtain the change of the battery voltage, for example, the battery voltage is sampled at every predetermined sampling period. Consecutively sampled voltages Vi to V0 are temporarily stored in a memory and the change of the battery voltage is computed by subtracting the i-th sampled voltage V1 from the currently sampled voltage V0. Therefore, the change of the battery voltage cannot be obtained until a period of time corresponding to i sampling periods has been expired. The battery pack that has reached the fully charged condition during this period of time cannot be detected as being fully charged. The same is true with respect to detection of the fully charged condition based on the change of the battery temperature.
In addition, when the cordless power tool is frequently switched ON and OFF during charging the battery pack, the charging circuit is rendered OFF and ON corresponding to ON and OFF of the cordless power tool. For a circuit configuration in which a relay contact is used for switching the charging circuit ON and OFF, the number of switching actions of the relay contact is increased if the cordless power tool is frequently switched ON and OFF, causing the service life of the relay contact and of the DC power source unit to shorten.
In view of the foregoing, it is an object of the invention to solve the above-described problems accompanying the prior art devices and to provide a DC power source unit that can accurately detect the fully charged condition of a battery pack and has a long service life.
To achieve the above and other object, there is provided according to one aspect of the invention a DC power source unit that is used as a power source of an electrically powered tool. The DC power source unit includes charging means, a power output switch circuit, a battery condition detecting means, and a controller. The charging means is provided for charging the battery pack used as an alternative power source of the tool. The power output switch circuit is provided for switching between supplying the DC power to the tool and supplying DC power to the battery pack for charging the battery pack. The battery condition detecting means is provided for detecting a condition of the battery pack and outputs a condition signal indicative of the condition of the battery pack. The controller receives the condition signal from the battery condition detecting means and determines that the battery pack is fully charged when the condition signal has reached a predetermined value. The controller determines that the battery pack is fully charged regardless of whether the charging means is supplied with the DC power. In one example, the battery condition detecting means is battery temperature detector. The battery temperature detector detects a temperature of the battery pack. In this case, the condition signal output from the battery temperature detector is indicative of the temperature of the battery pack. According to the invention, even if the DC power is supplied to the tool for its operation and the battery pack is not charged, the controller determines that the battery pack has reached the fully charged condition based on the temperature of the battery pack.
Measuring means may be further provided for measuring a non-use period of time during which time the tool is continuously held in a non-use condition. The controller outputs a charge enabling signal to the power output switch circuit when the non-use period of time exceeds a predetermined period of time to allow the charging means to resume charging the battery pack. Conversely, when the non-use period of time is shorter than the predetermined period of time, the charging the battery pack is prohibited.
Load current detecting means may further be provided for detecting a load current flowing in the tool. The controller determines that the tool is in the non-use condition when the load current detecting means does not detect the load current.